The present invention relates generally to the field of telemetry systems for transmitting information through a flowing stream of fluid. More particularly, the invention relates to the field of mud pulse telemetry where information detected at the bottom of a well bore is transmitted to the surface by means of pressure pulses created in the mud stream that is circulating through the drill string. Still more particularly, the invention relates to an acoustic signal detector that senses the pressure pulses in a bypass loop outside the main mud supply line.
Drilling oil and gas wells is carried out by means of a string of drill pipes connected together so as to form a drill string. Connected to the lower end of the drill string is a drill bit. The bit is rotated and drilling accomplished by either rotating the drill string, or by use of a downhole motor near the drill bit, or by both methods. Drilling fluid, termed mud, is pumped down through the drill string at high pressures and volumes (such as 3000 p.s.i. at flow rates of up to 1400 gallons per minute) to emerge through nozzles or jets in the drill bit. The mud then travels back up the hole via the annulus formed between the exterior of the drill string and the wall of the borehole. On the surface, the drilling mud is cleaned and then recirculated. The drilling mud is used to cool the drill bit, to carry chippings from the base of the bore to the surface, and to balance the hydrostatic pressure in the rock formations.
When oil wells or other boreholes are being drilled, it is frequently necessary or desirable to determine the direction and inclination of the drill bit and downhole motor so that the assembly can be steered in the correct direction. Additionally, information may be required concerning the nature of the strata being drilled, such as the formation's resistivity, porosity, density and its measure of gamma radiation. It is also frequently desirable to know other down hole parameters, such as the temperature and the pressure at the base of the borehole, as examples. Once these data are gathered at the bottom of the bore hole, it is typically transmitted to the surface for use and analysis by the driller.
One prior art method of obtaining at the surface the data taken at the bottom of the borehole is to withdraw the drill string from the hole, and to lower the appropriate instrumentation down the hole by means of a wire cable. Using such "wireline" apparatus, the relevant data may be transmitted to the surface via communication wires or cables that are lowered with the instrumentation. Alternatively, the instrumentation may include an electronic memory such that the relevant information may be encoded in the memory to be read when the instrumentation is subsequently raised to the surface. Among the disadvantages of these wireline methods are the considerable time, effort and expense involved in withdrawing and replacing the drill string, which may be, for example, many thousands of feet in length. Furthermore, updated information on the drilling parameters is not available while drilling is in progress when using wireline techniques.
A much-favored alternative is to employ sensors or transducers positioned at the lower end of the drill string which, while drilling is in progress, continuously or intermittently monitor predetermined drilling parameters and formation data and transmit the information to a surface detector by some form of telemetry. Such techniques are termed "measurement while drilling" or MWD. MWD results in a major savings in drilling time and cost compared to the wireline methods described above.
Typically, the down hole sensors employed in MWD applications are positioned in a cylindrical drill collar that is positioned close to the drill bit. The MWD system then employs a system of telemetry in which the data acquired by the sensors is transmitted to a receiver located on the surface. There are a number of telemetry systems in the prior art which seek to transmit information regarding downhole parameters up to the surface without requiring the use of a wireline tool. Of these, the mud pulse system is one of the most widely used telemetry systems for MWD applications.
The mud pulse system of telemetry creates acoustic signals in the drilling fluid that is circulated under pressure through the drill string during drilling operations. The information that is acquired by the downhole sensors is transmitted by suitably timing the formation of pressure pulses in the mud stream. The information is received and decoded by a pressure transducer and computer at the surface.
In a mud pressure pulse system, the drilling mud pressure in the drill string is modulated by means of a valve and control mechanism, generally termed a pulser or mud pulser. The pulser is usually mounted in a specially adapted drill collar positioned above the drill bit. The generated pressure pulse travels up the mud column inside the drill string at or near the velocity of sound in the mud. Depending on the type of drilling fluid used, the velocity may vary between approximately 3000 and 5000 feet per second. The rate of transmission of data, however, is relatively slow due to pulse spreading, modulation rate limitations, and other disruptive forces, such as the ambient noise in the drill string. A typical data bit rate is on the order of a bit per second. Some present day systems operate at higher frequencies, for example, 3 bits per second, and up to 10 bits per second with data compression. Representative examples of mud pulse telemetry systems may be found in U.S. Pat. Nos. 3,949,354, 3,958,217, 4,216,536, 4,401,134, and 4,515,225.
Mud pressure pulses can be generated by a number of known means which operate downhole to momentarily divert or restrict the mud flow. Without regard to the type of pulse generation employed, detection of the pulses at the surface is sometimes difficult due to attenuation of the signal and the presence of noise generated by the mud pumps, the downhole mud motor and elsewhere in the drilling system. Present day detectors employ one or more pressure transducers to detect the mud pulses. The transducers detect variations in the drilling mud pressure at the surface and generate electrical signals responsive these pressure variations. The pressure transducer is typically mounted directly on the line or standpipe that is used to supply the drilling fluid to the drill string. An access port or tapping is formed in the pipe, and the transducer is threaded into the port. With some types of transducers, a portion of the device extends into the stream of flowing mud where it is subject to wear and damage as a result of the abrasive nature and high velocity of the drilling fluid.
In another present day apparatus for detecting pressure pulses, the internal fluid passageway in the mud supply line is constricted at a particular location such that the drilling fluid must pass through adjacent regions having different cross sectional areas. This is accomplished by cutting and removing a segment of the supply line at the predetermined location. The removed section of pipe, which typically may be 8 inch diameter rigid metal pipe approximately 24 inches long, is then replaced with a generally tubular body that has been machined to include the desired reduced area portion. The body of such a detector includes a through bore for conducting the drilling fluid and typically has an outside diameter approximately the same size as the piping comprising the mud supply line. The body further includes an access port into the internal passageway at each of the regions of differing cross sectional areas. The body is welded into the supply line in place of the removed pipe segment, and each of the ports is then interconnected by a conduit to a different input port of a differential pressure transducer. The acoustic signal carded by the flowing drilling mud induces an added velocity component to the drilling mud passing through the body. The venturi effect produced in the mud by the constriction in the flow line amplifies the pulsing acoustic velocity signal, and the increased pressure signal is detected by the differential pressure transducer. While the use of venturi effects in obtaining steady flow rates from steady differential pressure measurements is known, the extrapolation of transient, compressible signals from similar measurements is not. Also, because this detector measures differential and not absolute pressure, it is relatively insensitive to many of the common sources of extraneous pressure pulses or "noise" that may arise during drilling by, for example, the drill bit becoming stuck and unstuck, or slipping and sliding in the hole.
While a detector using a differential pressure transducer and the in-line flow constrictor described above has proven useful in certain applications, the detector has certain inherent disadvantages. First, the flow constrictor adds additional power requirements due to the fact that the same volume of mud must now be pumped through the constriction. Further, the in-line constrictor body is heavy and cumbersome to transport and install. The installation requires that the mud supply line be cut in two places, and that the constrictor body then be welded in place. These procedures often prove difficult and time consuming. The difficulties are compounded when the procedures must be carried out under adverse weather conditions.
Additionally, because the body is installed "in-line," it carries the full flow of drilling mud, which frequently includes abrasive materials. The resulting erosion inside the constrictor body may require that the body be replaced periodically. Changing out the body is as complicated and time consuming as the original installation. In an attempt to lengthen the useful life of the constrictor body, a special hardfacing material has sometimes been applied to the internal surfaces of the body to reduce erosion and delay replacement. Such special treatment, however, adds significant expense to the manufacturing cost such a detector.
Thus, while it is advantageous to obtain information regarding the operating parameters and environmental conditions of the drill bit and motor using a flow constrictor and differential pressure transducer as described, there remains a need in the art for a detector that is insensitive to many of the extraneous pressure signals generated during drilling operations and, at the same time, does not require the same invasive and difficult procedures for installation. Preferably, the detector would be relatively small and light weight, easily transported and simple to install. Ideally, the detector components would operate outside of the main mud flow path, and thus would not require that expensive hardfacing materials be used in their manufacture.